1. Field of the Invention
The present invention relates to an air heater for heating intake air in an air cleaner for an internal combustion engine.
2. Description of the Related Art
When the temperature of intake air supplied from an air cleaner included in an intake system for an internal combustion engine mounted on a vehicle is excessively low, intake air has a large density. Consequently, it is difficult to atomize fuel in such high-density intake air, the exhaust gas discharged from the internal combustion engine has a large hydrocarbon concentration and the output of the internal combustion engine decreases.
An air heater for heating intake air in an air cleaner is proposed in JP-U No. 1-173449. This prior art air heater is used in combination with an air cleaner having an air cleaner case, and an air cleaner element laterally dividing the interior of the air cleaner case into a dust chamber provided with an air inlet and a clean chamber provided with an air outlet. The air heater has a heat radiator disposed in an air passage in the air cleaner case, and the cooling water or the engine oil is circulated through the heat radiator. Air sucked through the air inlet formed in a side wall of the dust chamber is filtered by the air cleaner element, and clean air filtered by the air cleaner element flows from the clean chamber through the air outlet toward the internal combustion engine.
An example of the prior art air heater has an oil-heated heat radiator formed by attaching a plurality of heat radiating fins to a pipe and disposed in the dust chamber of the air cleaner case. The engine oil is circulated through the oil-heated heat radiator. Intake air is thus heated in the air cleaner to avoid the reduction of engine output when the temperature of the atmosphere is low.
The heat radiator is placed in a heating medium circulating passage provided with a manual valve. When necessary, the heating of intake air is stopped by closing the manual valve to prevent the reduction of engine output, the occurrence of knocking and the increase of fuel consumption due to the excessive rise of intake air temperature.
The heat radiator of the heating device is combined integrally with the air cleaner and it is difficult to attach the heat radiator to and remove the same from the air cleaner. This prior art air heater is designed specially for an air cleaner with air heater and cannot be incorporated into an ordinary air cleaner originally not provided with any air heater. When intake air flows through the air cleaner, intake air must necessarily flow through the heat radiator even when intake air need not be heated, and the heat radiator imparts a resistance against the flow of intake air. Since the manual valve placed in the heating medium circulating passage to control the operation of the heat radiator is operated manually, it is difficult to make the heat radiator function properly; that is, the manual valve may possibly be closed before intake air temperature rises to a level not requiring heating or the timing of closing the manual valve may possibly be delayed to heat intake air excessively.
The flow of a heating medium, i.e., the engine oil or the cooling water, through the heating medium circulating passage is controlled by the manual valve and the operation of the manual valve takes effect with a delay behind the operation of the manual valve. It is very difficult to operate the manual valve properly in expectation of a time lag between the time when the manual valve is operated and the time when the operation of the manual valve takes effect.
Since the interior of the air cleaner case is divided laterally into the dust chamber and the clean chamber by the air cleaner element, part of intake air in an upper region of the dust chamber is heated by the heat radiator and heated intake air flows through an upper part of the air cleaner element, while part of intake air in a lower region of the dust chamber is not heated sufficiently and the insufficiently heated intake air flows through a lower part of the air cleaner element. Consequently, the upper and the lower part of the air cleaner element differ from each other in filtering effect, the air cleaner element is unable to function efficiently because the same is clogged locally with dust, and the life of the air cleaner element is shortened.
The heat radiator, such as an oil-heated heat radiator or water-heated heat radiator, placed in the dust chamber is formed by attaching heat radiating fins to bent pipe. The heat radiator tends to disturb the flow of intake air, which is one of causes reducing the air cleaning efficiency of the air cleaner element.